Chromium plating is the coating of choice for many metal finishing applications and demand for chrome's bright and lustrous finish continues to grow. Chromium has withstood competitive challenges from other finishes due to its unmatched aesthetics as well as its superior technical capabilities, including corrosion performance and multi-substrate capability. Chromium is widely used in the metal finishing industry for both decorative and hard chrome plating.
Chromium is traditionally electroplated from electrolytes containing hexavalent chromium, but many attempts over the last fifty years have been made to develop a commercially acceptable process for electroplating chromium using electrolytes containing only trivalent chromium ions. The incentive to use electrolytes containing trivalent chromium salts arises because hexavalent chromium presents serious health and environmental hazards. The waste from a hexavalent chromium based solution creates significant environmental concerns and hexavalent chromium baths require special treatment prior to disposal to comply with regulations. Thus, hexavalent chromium ions and solutions from which hexavalent chromium can be plated have technical limitations including the ever-increasing cost of disposing of plating baths and rinse water.
Trivalent chromium plating solutions have become an increasingly popular alternative in the metal finishing industry to hexavalent chromium plating solutions for a variety of reasons, including increased throwing power, as well as lower toxicity. The total chromium metal concentration used in a trivalent chromium solution is also significantly less than that of a hexavalent plating solution, and this reduction in metal, in addition to a lower viscosity of the solution, leads to less dragout and wastewater treatment. Trivalent chromium baths, as a result of their excellent throwing power, also typically produce less rejects and allow for increased rack densities as compared with hexavalent chromium baths.
The trivalent chromium plating rate and hardness of deposit are also similar to that of hexavalent chromium and trivalent chromium electrolytes also operate in the same temperature range as hexavalent chromium electrolytes. However, trivalent chromium electrolytes tend to be more sensitive to metallic impurities than hexavalent chromium electrolytes. Impurities can be removed by means of ion exchange or by precipitating agents followed by filtration.
The two main bath chemistries for trivalent chromium electrolytes are based on chloride and sulfate. In some instances, sulfate-based systems are more beneficial than chloride-based systems for a variety of reasons. For example, the deposit from a sulfate-based system has a higher purity, which leads to better corrosion protection and a color closer to that of hexavalent chromium. The chemistry of the sulfate-based systems is also less corrosive, which prevents deterioration of the plating environment and component areas.
Historically, the color of trivalent chromium deposits has been darker than that of hexavalent chromium deposits. While this problem has been greatly reduced, there are still some slight color differences between the two finishes. Trivalent chromium deposits are essentially produced in two forms—the first form is that which simulates, as closely as possible, the color of hexavalent chromium, and the second form is that which are specifically designed to give a different color to produce a desired cosmetic finish effect.
In addition, dark trivalent chromium coatings are becoming more popular in the industry. The appearance of a dark and shiny finish that can withstand the testing criteria of hexavalent chromium is desirable for many applications and dark trivalent chromium solutions have been developed that meet both appearance and technical requirements. It is desirous for these solutions to exhibit excellent covering and throwing power, consistent color at a wide range of current densities and the advantage of low-metal operation in comparison to hexavalent chromium.
Color additives can be difficult to analyze and control and thus color consistency can be difficult to achieve. It is desirable to provide a means for analyzing and controlling the color of trivalent chromium deposits to maintain color consistency of the deposits.